The present invention is directed, in general, to a method and apparatus for tracking the drilling of boreholes at a substantial depth in the earth, and more particularly to methods for determining the relative location of a reference well from a borehole being drilled through the use of a beacon located on the reference well casing.
The difficulties encountered in tracking and guiding the drilling of a borehole that is intended to intersect, to avoid, or to drill on a precise predetermined path to, a reference well at great depth below the surface of the earth are well known. Such guidance may be required, for example, when it is desired to construct a complex underground “plumbing system” for the extraction of underground gas, oil or bitumen deposits. Various electromagnetic methods for the precise drilling of such boreholes have been developed and have met with significant success during the past few years. Such methods and the instruments used are described, for example, in U.S. Pat. No. 4,323,848 and in U.S. Pat. No. 4,372,398, both issued to the applicant herein, and in U.S. Pat. No. 4,072,200 issued to Morris, et al. See, also, Canadian Patent 1,269,710 to Barnett et al, issued May 29, 1990.
Even though the guidance of boreholes with respect to existing wells is, in general, well developed, special problems can occur where existing techniques are not sufficient to provide the precise control that is required for that situation. For example, when it is desired to locate and to either avoid or to intersect a particular target well in a field that includes numerous other wells, problems can occur. Such a situation can occur when multiple wells lead from wellheads at a single location, such as a drilling platform, and it becomes necessary to drill a new borehole that avoids intersecting neighboring wells or, alternatively, to drill a new well for the purpose of intersecting a particular one. In this case, all the wells start at approximately the same location and spread downwardly and outwardly from each other. The new borehole being drilled may start at the same general location as the other wellheads, or may start at a location several hundred feet from the wellhead of a target well, and if intersection with, or avoidance of, a specific well, is desired, the problems of distinguishing between wells can be daunting.
Problems of tracking and guidance are also encountered when drilling non-parallel wells, such as drilling a horizontal well through a field of vertical wells, or vice versa, where it is desired to avoid the existing wells, or, in the alternative, where it is desired to intersect a specific well. Another area of difficulty occurs in the drilling of multiple horizontal wells, particularly where a well being drilled must be essentially parallel to an existing well. The need to provide two or more horizontal wells in close proximity, but with a precisely controlled separation, occurs in a number of contexts, such as in steam assisted recovery projects in the petroleum industry, where steam is to be injected in one horizontal well and mobilized viscous oil is to be recovered from the other. This process is described, for example, in Canadian Patent No. 1,304,287 of Edmunds et al, which issued Jun. 30, 1992. Another example is in the field of toxic waste disposal sites, where parallel horizontal wells are needed so that air can be pumped into one and toxic fluids forced by the air into the other for recovery. Still another example is in hot rock geothermal energy systems, where there is a need to drill parallel wells so that cold water can be injected into one and heated water recovered from the other. A further example is the drilling of boreholes for the pipeline industry, where the problem of connecting boreholes underground requires precise homing in from boreholes drilled, for example, from the opposite sides of a river.
The need to drill horizontal, parallel wells is of most immediate concern in the mobilization of heavy oil sands, where a borehole is to be drilled close to and parallel to an existing horizontal well with a separation of about five meters for a horizontal extension of a thousand meters or more at depths of, for example, 500 meters or more. A number of such wells may be drilled relatively closely together, following the horizon of the oil producing sand, and such wells must be drilled economically, without the introduction of additional equipment and personnel.